The present invention is directed to a polymer containing ionic functionality which produces an extremely thin, that is, ultra thin, coating which is preferably a surface coating layer or film. The prior art has failed to teach or suggest such a polymer or polymer coating, in particular, on an oppositely charged substrate, which can be used as a membrane. We have discovered that improved flux and selectivity characteristics are obtained with our membrane compared to known membranes. Unlike known membranes, our membrane is formed by coating a charged substrate with an oppositely charged polymer coating to form a substantially neutral interface between coating and substrate.
U.S. Pat. No. 3,083,118 to Bridgeford contains shot-gun teachings concerning a polymer-modified material with anion and cation-exchange sites. This material is used to form thick, charged films. Monomers that are polymerized are, for example, styrene butyl acrylate and vinyl chloride. A multi-component catalyst system is used: Fe.sup.++ /H.sub.2 O.sub.2. The reference further generally relates to monomers containing tertiary alkyl groups and alkyl amines used to make polymers containing ion-exchange groups. However, our invention is directed to a membrane product having an ultra thin coating layer which obtains improved selectivity and flux. These improved characteristics are not obtained using thick, charged films.
U.S. Pat. No. 3,647,086 contains disclosure relating to a cation-exchange membrane, where primary and secondary amines, including dodecylamine, are reacted with sulfonated polystyrene on a neutral substrate with subsequent conversion of free amine groups into acid amide bonds. However, this is not our invention.
Quaternizing a halogenated polystyrene with tertiary amine containing alkyl groups with twelve or more carbon atoms to form microporous or ion-exchange membranes is known, by U.S. Pat. Nos. 3,852,224 and 4,262,041, respectively.
It is also known to form ion-exchange membranes having a thickness of as small as 0.5 microns by dipping, coating or spraying a polymer on a substrate, by U.S. Pat. Nos. 3,510,418; 3,945,927 and 4,360,434. However, our invention relates to an ultra-thin coating layer, having a thickness substantially less than 0.5 micron, on a substrate to form a membrane having a substantially neutral interface between coating and substrate.
Composite membranes, formed by coating a porous substrate with a thin film, are known in the art by U.S. Pat. Nos. 3,808,305 and 4,125,462. Substrates include ultrafiltration, reverse osmosis or electrodialysis membranes or other conventional filtration materials.
As mentioned, the present invention relates to a polymer containing ionic functionality useful in forming an ultra-thin lipid layer which can be used to constitute a membrane, having a substantially neutral interface between coating and substrate. The polymer, method of using the polymer, and the coating and the membrane product thereof, having improved selectivity and flux characteristics, distinguish from the background art.